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[COLOR=darkgreen][B]No.8[/B][/COLOR][COLOR=black]: What's the difference between DPI and PPI?[/COLOR]
[B][COLOR=darkgreen]DPI[/COLOR][/B] stands for '[B][COLOR=darkgreen]Dots Per Inch[/COLOR][/B]'. It's often confused with [COLOR=darkgreen][B]PPI[/B][/COLOR], or '[B][COLOR=darkgreen]Pixels Per Inch[/COLOR][/B]'.
DPI is all about the printed representation of an image, so in general, several printed dots are required to represent a single image pixel.
PPI describes the number of image pixels that are displayed in an inch of the display medium, like a computer screen, for example.
My LCD monitor's screen is 20.5 inches wide, and its horizontal resolution is 1920 pixels. Divide the pixels by the inches and you get the PPI, which - in this case - is just under 94 PPI.
If you were to print a 4288 pixel (longest side) wide image onto a 12x16 inch sheet of photo paper, by dividing the pixels by the 16 inches, you get a PPI figure of 268.
But let's go back to DPI. Your printer might have a printing resolution of 1440 DPI, or 1,440 printer dots per inch in each x/y dimension. Divide the PPI figure for the image printed to the paper size example above and you will see that (1440 divided by 268) about 5.3 dots are used to cover the distance that one printed image pixels covers.
Of course an image pixel is square, so each image pixel requires 5.3 squared printer dots, or 28 printer dots per image pixel.
In practice, it's advisable to ensure that your image PPI figure divides perfectly into the printer DPI. This avoids the need for the printer to have to interpolate, or fill in gaps, with extra dots, which can result in moir� patterns and strange artifacts, especially along high contrast edges.
So taking the example of 1440 DPI on the printer side, you might want to ensure that your image PPI is, for example, 288, which is exactly one fifth of 1440.
Incidentally, as a rule of thumb, you should try to ensure that your image is printed at a size that doesn't go below 200 pixels per printed inch, unless the print is fairly small to start with.
You can change the printed resolution of your image by [B][COLOR=darkgreen]re-scaling[/COLOR][/B] the image without '[COLOR=darkgreen][B]re-sampling[/B][/COLOR]' - so your change the number of printed pixels per inch (PPI) without affecting the actual number of pixels in the image.
[B][COLOR=darkgreen]Tomorrow:[/COLOR][/B] Why do we need lens coatings?
[I]Incidentally, please don't hesitate to post a question about this Daily photo tip if you have one![/I]
I would like to add a view on the subject of what is the optimum PPI to send to the
printer.
In the various writings by acknowledged experts on printing workflows, including an unnamed Epson employed scientist, the printer's algorithm for translating, say 300 ppi, to the printers 1440 dpi, is so complex that the input ppi is almost irrelevant if it is above 180ppi. There is therefore no need to undertake complex calculations to work out the best ppi for any given print. Any resolution between 180 and 480 ppi is suggested by Jeff
Schewe who is the Epson specialist, Michael Reichmann, and others.
I do all my printing through Qimage which removes all need to change resolution for any sized print, and which also produces better prints by far than Lightroom or Photoshop!
Qimage also removes the need to re-size your images for printing, so for the last eight years all my nefs and dng images are stored at their original size, and the only time I make a re-sized copy is for web work.
I realise that this will probably start an argument, but I challenge anyone who hasn't tried Qimage yet to do so.
Download the trial, prepare a print for say 8x12 re-sized in Photoshop, and print it at your chosen resolution through PS. Then using Qimage print your originally
prepared picture at 12x8 but without re-sizing. I guarantee that you will see a difference in print quality.
The interpolation done by Qimage is exceptionally good, even though I don't know how it works. For example, using a full nef file, 4288 x 2848 pixels of 10MB:
Print a 6 x 4 borderless Qimage sends 711 ppi as 28MB
.. 9 x 6 .. .. 475 ppi as 60MB
.. 12 x 8 .. .. 285 ppi as 165MB
Of course I must disclaim any interest in Qimage other than as a very satisfied user for 8 years
Roger